Partially shielded food package for dielectric heating

ABSTRACT

A food package comprising a container of dielectric sheet material and a conductive sheet thereon. The conductive sheet is of smaller area than the dielectric sheet and is shaped and oriented in relation to the package wall configuration to expose a plurality of food articles therein differentially to microwave radiation.

1451 Feb. 11,1975

United States Patent 1 Pothier et al.

[54] PARTIALLY SHIELDED FOOD PACKAGE 3.079.912 3/1963 426/107 FOR DIELECTRIC HEATING 426/107 Krajewski [75] Inventors: Robert G. Pothier, Amherst, N.H.; Thomas E. Ford, Arlington, Mass.

[73] Assignee: Trans World Services, Inc., Melrose, Primary Christian Mass. Attorney, Age/1!. or FirmKenway & Jenney [22] Filed: Nov. 15, 1973 [21] Appl. No.1415,962

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FIG. 5E

FIG. 50

PARTIALLY SHIELDED FOOD PACKAGE FOR DIELECTRIC HEATING BRIEF SUMMARY OF THE INVENTION This invention relates generally to shielded containers for selective and controlled heating or cooking of foods in microwave ovens. More particularly, it relates to containers for a plurality of food articles that are to be heated or cooked to a differing extent.

A principal object of this invention is to provide a container in which all of the principal ingredients of a sandwich-type food product may be included, and which may be placed in a microwave heating oven so as to expose the ingredients to the high frequency field, in such manner that each ingredient is properly heated or cooked. For example, a hamburger may be thoroughly heated to a cooked, sizzling state, whereas the bun will be only warmed. Many other examples of use for such containers will occur to those engaged in the food service and vending industries.

Another object of the invention is to accomplish the necessary heating or cooking without arcing or charring of the foods or the container. Arcing is a plasma arc discharge that produces aflash of light, a noise, and sometimes ignition of the container, where the same is constructed of paper, plastic or other flammable material. A major cause of arcing is the conductive shield which in some instances is placed adjacent the food articles to protect the same from excessive heating in the dielectric field. Any deviation of a conductive shield edge from a straight line produces an intensification of the electric field emanating from that edge. For any irregularity having a dimension significantly less than one quarter wavelength, the longer and the more pointed it is, the stronger the field strength associated with it. At locations where the field strength is sufficiently large, an arc discharge will occur, and the heat developed in the arc may be sufficient to cause browning or burning of adjacent portions of the food articles or the container. If the container is thermoplastic, it may deform or melt. In severe cases a paperboard container may char badly, and a hole may be produced.

Closely related to arcing is charring, which is less severe, but which may result in browning of container materials or charring thereof.

Charring and arcing may be influenced by other factors such as resonant shield geometries, standing wave patterns in the microwave oven, the oven power level, and the quantity of food exposed to the microwave radiation.

The objects of this invention=include the provision of food packages so constructed as to minimize the effects of arcing and charring, while providing for uniform, controlled heating and cooking of each of the food items contained within the package.

Other objects of the invention will be appreciated from the following detailed description.

With the foregoing objects in view, the features of the invention reside in the provision of a food package comprising a container of dielectric sheet material and a conductive sheet thereon. The conductive sheet is of smaller area than the dielectric sheet and is shaped and oriented in relation to the package wall configuration so as to minimize arcing and charring while properly exposing food articles in the package differentially to microwave radiation. The manner of shaping and orienting the conductive sheet is a function of the package configuration and of the food articles therein, as will become evident from the following description.

BRIEF DESCRIPTION OF THE DRAWING FIG. I is a plan view ofa box blank embodying a first form of the invention suitable for use with hamburgers to be heated or cooked in a microwave oven.

FIG. 2 is a view in perspective showing the blank of FIG. 1 in set up form.

FIG. 3 is an elevation in section of the box of FIGS. I and 2 containing the ingredients of a hamburger, as oriented on the shelf of a microwave oven.

FIG. 4 is a view in perspective of a box containing a frankfurter, representing a second embodiment of the invention.

FIGS. 5A to SF are schematic illustrations of a plurality of embodiments representing different food package configurations.

FIG. 6 is a view illustrating the application of the invention to the cooking or heating of liquid or semiliquid food ingredients.

DETAILED DESCRIPTION FIG. 1 illustrates a first embodiment of the invention suitable for heating or cooking hamburgers. The package comprises a blank 12 formed of a sheet 14 of dielectric material, preferably but not necessarily of paper sufficiently stiff to form a box 16 as shown in FIGS. 2 and 3, and a one-piece conductive sheet or shield 18 laminated to the sheet 14 to cover portions thereof. Preferably, the conductive sheet 18 is a metallic foil such as aluminum or tin. The conductive sheet is preferably but not necessarily attached to the side of the dielectric sheet 14 that forms the inside of the box 16. It will be understood, however, that while the sheet 18 is preferably laminated on the sheet 14, this invention also contemplates other means of putting the conductive sheet on the dielectric sheet such as chemical deposition, silver painting, vacuum metallic deposition and any other known equivalent techniques.

For use in a vending machine, the box 16 is filled with a pair of roll halves 20 and 22, over which a meat patty 24 is placed in a position adjacent to a cover or closure 26 formed by a panel 28 of the blank 12. With the cover 26 closed, the box 16 is placed on a shelf 29 in a microwave oven with the cover in contact with the shelf. As further explained below, in this position the meat patty is located in a relatively unshielded region of the box and is exposed to microwave energy from the direction of the shelf 29. On the other hand, the roll halves 20 and 22 are more remote from the shelf and are relatively protected by portions of the conductive sheet 18 on the sides other than that adjacent to the meat patty. As a result, the meat patty develops a substantial amount of heat and may be heated to a sizzling, cooked condition while the roll halves are only warmed. After a few seconds the package 16 may be removed from the oven, the cover 26 opened and the contents rearranged so that the roll half 20 may be placed over the meat patty to complete the sandwich in edible form.

The blank 12 may be formed in any of several configurations well known in the box making art, the preferred form being illustrated in FIG. 1. The sheet 14 comprises a number of panels defined by edges, score lines and fold lines so as to form the six-sided box 16.

3 The foil sheet 18 comprises portions 30, 32, 34, 36 and 38, the four last-mentioned portions having right angle folded flaps 40. The edges of the sheet 18 are substan tially spaced from three-dimensional corner points 42 of the box, and they are also substantially spaced from the corner lines 44 when the box is in the set up position as illustrated in FIG. 2. The edge of the sheet 18 is also smoothly rounded adjacent the corner points 42 as most clearly shown by fillet-like curves at 46 in FIG.

It will be further observed in FIG. 2 that when the box is set up, the most closely adjacent peripheral edges 48 of the flaps 40 are substantially spaced from one another, as well as being smoothly rounded. Moreover, as shown in FIG. 1, at the fold lines such as 50 adjacent to the flaps 40, the edges of the sheet 18 have smoothly rounded indentations such as.52.

As shown in FIG. 3, when placed on the shelf 29 of the oven, the substantially unshielded cover 26 spaces the flaps 40, which are parallel to the shelf, from the shelf. This spacing may be simply the thickness of the dielectric sheet, or it may be made substantially greater than such thickness by suitable configuration of the cover 26.

The object of these structural features is to minimize charring and arcing, while permitting differential exposure of the package contents to microwave energy. This is explained in further detail as follows.

It has been found desirable to reduce the number and sharpness of points in the conductive sheet 18, which tend to increase the strength of the electrical field in a microwave oven. To a maximum extent, all corners are rounded, as illustrated for example by the inside corners 46 and the outside corners 48. Rounding of the corners of the conductive sheet'l8 is carried out with respect to two-dimensional corners as well as threedimensional corners, as illustrated. Such rounding is carried out by suitable shaping of the edge contours of the sheet 18, whether or not the dielectric sheet 14 is provided with relatively sharp corner points and corner lines, since the latter are non-conductive and of rela-- tively less importance to the phenomena of charring and arcing.

The portions of the conductive sheet 18 adjacent the shelf 29, because of this proximity, are possible sites of arcing or charring. It has been found that, in general, a conductive edge perpendicular to the shelf in a microwave oven is likely to produce these phenomena. It has been discovered that these phenomena may be inhibited or reduced by the features illustrated in the embodiment of FIG. 1. Thus, the folding over of portions of the conductive sheet to form the flaps 40 parallel to the shelf results in reduced charring and arcing, as compared with an embodiment having no flap 40. Secondly, the spacing of the foil edges andflaps away from the shelf reduces charring and arcing, and in the illustrated embodiment this spacing is provided by the thickness of the closure or cover 26.

As previously noted, the illustrated embodiment of FIG. 1 provides an additional feature at points where the edge of the conductive sheet 18 is folded, as at fold lines such as 50. Indentations such as 52 provide curvatures at points of folding which reduce arcing and charrmg.

Moreover, providing sufficient distances between proximate extremities of the conductive sheet. such as the corners 48, further serves to reduce charring and arcing. v I

It will be noted that theentire conductivesheet 18 of the embodiment in FIG. I is a single integral metallic foil piece, with no overlapped joints. As compared with a structure in which the conductive sheet comprises plural pieces having a joint or seam, or one piece with an overlapped'joint,- the present embodiment is more char and arc resistant.

It is noted that the above-described features of the conductive sheet 18 result in unshielded regions or holes in the container through which microwave energy may leak. However, such leakage is not necessarily detrimental to the proper heating of the food, since the size and shape of the holes may be controlled to alter the standing wave pattern inside the shielded container, thereby producing heating nulls and peaks at desired locations therein. In general, by controlling the size, shape, number and location of the holes or unshielded regions, such desired results are obtainable.

It has been found that some food condiments, when smeared over shield extremities, will produce arcing. The high field strength associated with conductive extremities, if contiguous with semi-conductive condiments, promotes arcs. This may be avoided by locating shield extremities so that they are less likely to be smeared inadvertently by condiments for the food components in use.

It will be recognized that certain variations in the contents of the package of FIG. 1' may be readily accomplished. For example, to make a cheeseburger a slice of cheese is placed between the bun half 22 and the hamburger patty 24. In the event that other items are to be kept cool, such as lettuce, pickle chips or tomato slices, these are conveniently placed between the bun halves 20 and 22.

FIG. 4 illustrates a second embodiment of the invention, comprising a box 56 for a frankfurter 58 and sliced bun 60. This box is constructed in accordance with many of the principles described above in detail with reference to FIG.- 1. The blank is formed of a dielectric sheet 62 to form the panels of the box in accordance with existing box making techniques, and a conductive sheet 64 is laminated thereto, having portions covering a region of each of the walls of the box, except for the portions thereof adjacent to its corner points and corner lines, all as illustrated in FIGS. 1 and 2. A region of the box adjacent to a corner line 66 is unshielded and bounded by edges 68 that are smoothly curved to widen the center of the unshielded region intermediate its ends. Preferably, the frankfurter 58 is placed in the box adjacent to the unshielded region in the manner illustrated in FIG. SD.

The edges 68 of the conductive sheet or foil are shaped in the foregoing manner to provide for evenly heating the frankfurter 58 throughout its length. A meat frankfurter is a semi-conductor of electricity and therefore acts as a dipole antenna when heated in a microwave oven. For a standard length frankfurter that is not differentially shielded from end to end, the ends often overheat and the middle underheats. The resonant, standing waves produce heating peaks at the ends. The foil shield or conductive sheet 64, at the edges 68, illustrated in FIG. 4 compensates for this effect and produces an evenly heated frankfurter.

FIG. 6 illustrates a variant of the frankfurter sandwich in the form of what is sometimes called a submarine sandwich. This comprises an elongate bread roll or bun 70 and a plurality of meat balls or other particulate foods 72 with a soft or other semi-liquid filling (not shown) covering the meat balls. This filling is of such consistency that, when placed directly upon the bread roll 70, it would soak into the bread roll and produce an undesirable effect thereon. To obviate this effect. the meat balls 72 and sauce are placed within a tube 74 of plastic film. The tube is removed after heating in the oven. One end 76 of the tube is folded over to close it, while the other end is closed by a slotted, circular plastic squeegee 78. The tube 74 may be placed within a container such as that illustrated in FIG. 4 in substantially the same manner as the frankfurter 58.

FIGS. 5A to SC, 5E and SF illustrate alternative configurations of the conductive foil or sheet in relation to the contents, for example a frankfurter or other filling 80 and a bun or other bread product 82. In these drawings, outlines 84, 86, 88, 90 and 92 represent the appropriate configurations of the conductive foil or sheet, and it is assumed that the same is laminated to a dielectric sheet of larger area which is suitably constructed in accordance with known box making techniques to provide a closed or substantially closed container or box. In each of these embodiments a suitable unshielded region is provided, adjacent to which is located the particular food item to be most thoroughly heated or cooked. I

With respect to all of the container embodiments described hereinabove, the formation of the container blank may be followed by additional steps of fabrication, such as the application of glue lines or spots and the adhesion thereto of certain flaps and panels as preliminary steps. For example, in FIG. 1 shaped glue spots 94 may be placed upon a pair of flaps and adhered to adjacent panels of the box after folding upon a line 96. Other glue spots may also be applied, if desired. After the glue is set, the box may be folded flat for storage and shipment along creased lines 98. This facilitates setting up the collapsed box for the loading of sandwich ingredients without the further use of glue. Other techniques of the box making art may also be employed without departing from the spirit or scope of this invention.

We claim:

1. A food package comprising, in combination, a dielectric sheet and a conductive sheet thereon, said dielectric sheet having a plurality of panels and forming when set up a substantially closed container having a first wall and second and third walls contiguous with the first wall at fold lines intersecting at a corner point of the package. said conductive sheet having portions covering a region of each of said walls and-having an edge substantially spaced from said corner point.

2. A food package according to claim I, in which said second and third walls 'are mutually contiguous at a corner line of the package when set up, said edge of the conductive sheet being substantially spaced from said corner line throughout its length.

3. A food package according to claim 2, in which said edge is smoothly rounded adjacent said corner point.

4. A food package according to claim 1, in which the dielectric sheet has sufficient stiffness to form a selfsupporting box.

5. A food package according to claim 1, in which the conductive sheet is a metallic foil.

6. A food package according to claim 1 with a wall having an unshielded region uncovered by the conductive sheet for exposure toward a high frequency source, said package being adapted to contain a plurality of food articles with one of said articles being located more closely adjacent to said unshielded region.

7. A food package according to claim 6, in which the edges of the conductive sheet adjacent to said unshielded region are folded to form flaps substantially perpendicular to the direction of radiation from said source.

8. A food package according to claim 7 adapted to overlie the source, in which the dielectric sheet is adapted to space said flaps from said source.

9. A food package according to claim 1, in which the conductive sheet is continuous over a fold line between contiguous walls, and has a smoothly rounded indentation adjacent said fold.

10. A food package according to claim 1, in which the portions of the conductive sheet on said second and third walls are substantially spaced from one another when the package is set up.

II. A food package according to claim 6, in which the unshielded region-is elongate and bounded on its elongate sides by opposed edges of the conductive sheet, said last-mentioned edges being curved to widen the center of said unshielded region intermediate its ends. 

1. A food package comprising, in combination, a dielectric sheet and a conductive sheet thereon, said dielectric sheet having a plurality of panels and forming when set up a substantially closed container having a first wall and second and third walls contiguous with the first wall at fold lines intersecting at a corner point of the package, said conductive sheet having portions covering a region of each of said walls and having an edge substantially spaced from said corner point.
 2. A food package according to claim 1, in which said second and third walls are mutually contiguous at a corner line of the package when set up, said edge of the conductive sheet being substantially spaced from said corner line throughout its length.
 3. A food package according to claim 2, in which said edge is smoothly rounded adjacent said corner point.
 4. A food package according to claim 1, in which the dielectric sheet has sufficient stiffness to form a self-supporting box.
 5. A food package according to claim 1, in which the conductive sheet is a metallic foil.
 6. A food package according to claim 1 with a wall having an unshielded region uncovered by the conductive sheet for exposure toward a high frequency source, said package being adapted to contain a plurality of food articles with one of said articles being located more closely adjacent to said unshielded region.
 7. A food package according to claim 6, in which the edges of the conductive sheet adjacent to said unshielded region are folded to form flaps substantially perpendicular to the direction of radiation from said source.
 8. A food package according to claim 7 adapted to overlie the source, in which the dielectric sheet is adapted to space said flaps from said source.
 9. A food package according to claim 1, in which the conductive sheet is continuous over a fold line between contiguous walls, and has a smoothly rounded indentation adjacent said fold.
 10. A food package according to claim 1, in which the portions of the conductive sheet on said second and third walls are substantially spaced from one another when the package is set up.
 11. A food package according to claim 6, in which the unshielded region is elongate and bounded on its elongate sides by opposed edges of the conductive sheet, said last-mentioned edges being curved to widen the center of said unshielded region intermediate its ends. 